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希夏邦马峰地区始新世地壳加厚和隆升过程

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中新世中期之前喜马拉雅山脉的隆升历史和深部动力机制,还存在极大的争议、缺乏基本的数据约束.喜马拉雅造山带核部高级变质岩的埋藏和折返过程可以约束地壳加厚和山脉隆升过程,但是现有的变质记录主要为渐新世晚期-中新世中期,始新世变质作用的分布范围、形成过程和地质意义仍然未被充分挖掘.本文首先厘定了希夏邦马峰(8027m)山体的基本岩石组成:下部为高喜马拉雅正/副片麻岩,上部为肉切村群变粒岩及贯入其中的淡色花岗岩席.进一步对1964年希夏邦马峰登山科考采集的眼球状片麻岩进行了变质作用相平衡模拟和独居石微区原位U-Th-Pb定年,确定了始新世变质记录的P-T-t演化轨迹:M1阶段中P/T型变质压力峰期,条件为～1.0GPa、～740℃(～22℃/km),变质年龄为～37Ma;后近等温降压至M2阶段低P/T型变质温度峰期,条件为0.5～0.4GPa、～760℃(45～57℃/km),变质年龄为～25Ma;在～18Ma左右冷却至固相线以下.希夏邦马峰～37Ma的中P/T型变质和地壳加厚事件与北喜马拉雅穹隆、高喜马拉雅主体、前陆飞来峰等地区的记录一致(40～37Ma,20～25℃/km),表明印度-亚洲初始碰撞约20Myr后中上地壳被堆叠埋藏至＞33km以下,发生了广泛的整体性加厚和深熔作用.该事件与喜马拉雅内陆残留海最终退却的时间吻合,是喜马拉雅山脉初始隆升的诱因.喜马拉雅山脉不是晚新生代以来隆升的,而是经历了自始新世中期以来的长期地壳堆叠和隆升过程.
Eocene crustal thickening and uplift process of the Mount Shisha Pangma region
When and how the Himalaya uplifted before the Middle Miocene is still highly controversial and lacks basic data constraints.The burial and exhumation process of the Himalayan Metamorphic Core can constrain the above issues.However,the existing metamorphic records are mainly from the Late Oligocene to Middle Miocene.The distribution range,formation process and geological significance of the Eocene metamorphism and crustal thickening remain underexplored.This contribution first determines the basic rock assemblies of Mount Shisha Pangma(8027m):the lower part is composed of Greater Himalayan orthogneiss/paragneiss,and the upper part is composed of Rouqiecun Group leptynites and the injected leucogranite sills.Secondly,phase equilibrium modeling and monazite in situ U-Th-Pb dating of the augen gneiss collected during the Shisha Pangma Mountaineering Expedition in 1964 revealed the Eocene metamorphic record and P-T-t evolution:M1 stage medium P/T type pressure peak with the condition of～1.0GPa and～740℃(～22℃/km),and metamorphic age of～37Ma;It undergoes near-isothermal decompression to M2 stage low P/T type temperature peak with condition of 0.5～0.4GPa and～760℃(45～57℃/km)and metamorphic age of～25Ma;Finally,it cooled to sub-solidus temperatures at～18 Ma.The Eocene medium-pressure type metamorphism and crustal thickening at Shisha Pangma is consistent with records from the North Himalayan Gneiss Dome,the main Greater Himalayan Crystalline,and foreland klippes(40～37Ma,20～25℃/km).It indicates that～20Myr after the initial India-Asia collision,the middle and upper crust was buried to＞33km deep,causing extensive overall thickening and anatexis.This event coincided with the final retreat of the Himalayan inland relict sea and triggered the initial uplift of the Himalayan mountains.The Himalaya did not result from Late-Cenozoic uplift but experienced a long-term crustal stacking and uplift process since the Middle Eocene.

HimalayaInitial upliftCrustal thickeningMedium P/T type metamorphismMonazite U-Th-Pb petrochronology

王佳敏、侯康师、李潇丽、吴福元

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岩石圈演化国家重点实验室,中国科学院地质与地球物理研究所,北京 100029

中国科学院大学地球与行星科学学院,北京 100049

国家自然博物馆,北京 100050

喜马拉雅 初始隆升 地壳加厚 中P/T型变质作用 独居石U-Th-Pb岩石年代学

国家重点研发计划青藏高原综合科学考察研究项目(第二次)青藏高原综合科学考察研究项目(第二次)国家自然科学基金中国科学院青年创新促进会项目

2022YFF08008002019QZKK07032022QZKK0203424882012022065

2024

10.18654/1000-0569/2024.05.06

岩石学报
中国矿物岩石地球化学学会,中国科学院地质地球物理研究所

岩石学报

CSTPCD北大核心
影响因子：2.74
ISSN：1000-0569
年,卷(期)：2024.40(5)
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